The light transmitting performance of an optical fiber is highly dependent upon the properties of the polymer coating that is applied to the fiber during manufacturing. Typically a dual-layer coating system is used where a soft primary coating is in contact with the glass fiber and a harder secondary coating surrounds the primary coating. The harder secondary coating protects the fiber from damage caused by external forces and allows the fiber to be handled without concern of breakage. The softer primary coating dissipates forces that reach the interior of the coated fiber and prevent the forces from being transferred to the fiber. The primary coating is especially important in dissipating stresses that arise when the fiber is bent. The influence of bending stresses on the fiber needs to be minimized because bending stresses create local perturbations in the refractive index profile of the glass fiber that act to attenuate the intensity of light guided by the fiber. By dissipating stresses, the primary coating minimizes bend-induced attenuation losses.
Conventionally, certain characteristics have been desirable for the primary coating. For example, the modulus of the primary coating must be sufficiently low to cushion and protect the fiber by readily relieving stresses on the fiber. This cushioning effect must be maintained throughout the fiber's lifetime. The Young's modulus of the primary coating may generally be less than about 1.0 MPa, and more preferably, less than about 0.50 MPa.
Because of differential thermal expansion properties between the primary and secondary coatings, the primary coating should also have a glass transition temperature (Tg) that is lower than the foreseeable lowest use temperature. This enables the primary coating to remain elastic throughout the temperature range of use, facilitating differences in the coefficient of thermal expansion between the glass fiber and the secondary coating. Further, a low glass transition temperature enables the primary coating to remain soft when the fiber is deployed in a low temperature environment. The glass transition temperature of the primary coating may be less than about 0° C. Also, the tensile strength of the primary coating, which generally decreases as the modulus decreases, must be high enough to prevent tearing defects during on draw processing or subsequent processing of the coated fiber during cabling, etc. The tensile strength of the primary coating may generally be greater than about 0.3 MPa.